Current evidence suggests that protein phosphorylation and dephosphorylation are important biochemical mechanisms in the control of cardiac function and metabolism. The phosphorylation states of a number of proteins important in regulating cardiac contractility change in response to drugs and neurotransmitters which alter the contractile activity of cardiac muscle. The phosphorylation states of these proteins are regulated by the opposing actions of protein kinases and protein phosphatases. Although considerable work has been done on the protein kinases, very little is known about cardiac protein phosphatases and their roles in regulating cardiac function. The proposed studies involve analysis of cardiac phosphatases that dephosphorylate contractile proteins and are of fundamental importance for a more complete understanding of the control of protein phosphorylation in this tissue. The aims of these studies are to identify at the molecular level the structures, functions and regulatory properties of two major forms of cardiac protein phosphatase. The specificities of these enzymes toward a number of cardiac proteins including the inhibitory subunit of troponin, myosin light chains, myofibrillar C-protein, and proteins of the sarcoplasmic reticulum will be determined. Monoclonal and polyclonal antibodies will be used as specific probes to identify different forms of these phosphatases in subcellular fractions and to probe important functional and regulatory domains of these proteins. The methods of recombinant DNA and molecular genetics will be used to obtain detailed structural information about the two major cardiac phosphatases. Molecular genetic approaches will provide tools to address fundamental questions regarding the structures of different forms of this family of enzymes and their tissue specific expression. This will provide new information regarding the biological function(s) of these important regulatory enzymes.